Method for depolymerizing polystyrene



Patented Apr. 17, 1951 2,549,538 ICE.

. METHOD FOR DEPOLYMERIZING POLYSTYREN E William J. Sparks, Cranford, and David W. Young, Roselle, N. J assignors to Standard Oil Development Company, a corporation of Delaware N Drawing. Application August 25, 1945, Serial No. 612,722

4 Claims.

This invention relates to low temperature catalytic treatment of a synthetic resin. More specifically, it relatesto the formation and modification treatment of styrene polymers and simi- 'merization in a reasonable length of time, such as a number of hours, requires temperatures of 100 C. or higher, approaching the boiling point of the monomer 146 C. Still higher polymerization temperatures result in a more brittle polystyrene. The resulting resins were found to differ in strength, flowing qualities, solubility, and other properties related in some way to the structure and molecular weights of the polymers even though they appeared identical to the eye. On heating to temperatures above 300 C. polystyrene is depolymerized to the monomer.

Styrene polymers produced for commercial purposes have generally been required to have molecular weights in the range of about 30,000 to 200,000 regulated by conditions of formation, but much of the delay in commercializing styrenes, as these polymers are called, has been due to the difiiculty of control and of obtaining more rapid polymerization. The lower polymers (20,000 molecular weight and below) have low molecular strength and little commercial use.

Attempts were made to produce polystyrenes using stannic chloride and boronfiuoride at moderate temperatures above room temperature but the products were of very low molecular weight liquids (3,000 molecular weight and lower) Dee rivatives of styrene, such as alkyl substituted styrenes, and other homologues, or similar aryl alkene compounds, having an aromatic nucleus with an alkenyl or ethylenic side chain, were found to behave in a similar fashion to that of styrene but in general forming polymers of even lower molecular weight. These attempts were always unsuccessful for producing commercially useful resins, but now it is determined that the procedures used lacked certain important factors of control, such as adequate temperature reduction and adjustment, the proper use of solvents with an effective catalyst, and time control.

An object of this invention is to provide an improved method of catalytically producing high yields of modified styrene resins rapidly at low temperatures.

Another object of this invention is to provide new types of resins which have improved useful properties by a catalytic treatment of arylalkene polymers.

A further object of this invention is topiovide an efficient method of modifying resins at low temperatures to useful resins of selected molecular weights or intrinsic viscosities. Y

Attainment of the foregoing and further objects will be understood from the following description.

In accordance with the present invention, it is now ascertained that if the polymerizable aryl alkene monomers are kept in solution with an active Friedel-Crafts catalyst in a suitable solvent, while maintaining a very low polymerization temperature, e. g., -'78 C. or 100 C., the polymerization proceeds very rapidly and satisfactorily to give in about 3 to 10 minutes nearly 100% yields of high molecular weight polymers (e. g., about 80,000 molecular Weight and above) then on raising the temperature of the solution, if the polymers are kept in solution with the active catalyst, they become depolymerized and modified under control, as desired, in a reasonably short time.

The type of solvent, simple or mixed, must have the property of maintaining the polymers in solution at the modifying reaction temperature without deactivating the catalyst, i. e., so as not to form a deactivated catalyst complex. Such a solvent is of a limited type, preferably an alkyl halide, such as methyl or ethyl chloride, or methylene chloride. The low temperature solvents for the polymers may be mixed with a limited quantity of other liquid compounds to form suitable mixed solvents, even though said other compounds by themselves are not sufiiciently miscible at all low reaction temperatures. Liquefied C2 and C3 hydrocarbons, such as ethane, ethylene, and propane, may thus be mixed with an alkyl halide solvent.

Methyl chloride, having a boiling point of 23.7 C. and a melting point near l00 C., may be used as the solvent at polymerization temperatures from about C. to about 103 C. Dry methylene chloride having a boiling point of 39.8? C. and a freezing point of -97.6 C. may be used similarly. Ethyl chloride, having a boiling point of 122 C. and a melting point near 138 C., may be used at still lower temperatures, for example, with liquefied nitrogen as an external refrigerant, but greater dilution of the monomer and polymer is required in maintaininga homogeneous reaction solution at such lower temperatures. The low boiling hydrocarbons, such as propane and ethylene, cannot be used as solvents by themselves to obtain a homogeneous solution at temperatures below about -40 0.,

because they tend to precipitate the polymer from solution at lower temperatures.

ition.

3 A preferred catalyst-tor the polymerizationand modifying treatment is aluminum chloride, but other Friedel-Crafts type catalysts which are active and soluble in the required types of solvents may be used. The Friedel-JCraftscatalysts .merization. temperature :for a period ot about 3 to 10 minutes, the polymerization proceeds to iorm high molecular weight polymers.

" After the high molecular weight polymers have been formed zif they are allowed to remain include boron fluoride, AlBra, AlBrzCl, AlBrsCl, homogeneously in contact with the active catalyst [A12BI5C1'A1OBI], TiCl4, TiClsBr-and others listed andthe-temperature of the solution is raised, e. g., in the article by N. O. Galloway i'n-fiChemicalltetoabove ,'78.lC they undergo a depolymerizaviews published by the ,American Chemical tion and-modification. This modifying treatment Society at Baltimore in 1935, vol. XVII, No. 3, can be controlledto obtain new types of resins page 327. having desired characteristics.

A minimum suitable concentration of :the Fr or"the purpose of illustration, but without catalyst in liquid solvent is approximately 01%. limiting the invention, the following examples on An optimum concentration of aluminumbhloride the procedure and results are given: catalyst is above about {0.4 g. per 100 ml. to 0.8 5 .g./l00 ml in'methyl chloride, the maximumbeing EXAM} LE 1 approximately 6.0% .in ethyl ch fI'wo.literS-of.xC;rP.=styrene wereaddedztozth-ree In preparing the reaction Solutienfrom about liter of pure;methyl ch1oride. .The liquidzmixparts of diluent or solvent may be f f ture wasiplacedjnzajacketedcopperreactoriand with 1 p Of the monomeric reactant the 20 liquid ethylene was .contained ;in ,thejacket for mum-ratio being t required to e themono' externalcooling. when :the temperatureof .-.the meric reactant insolution at the polymerization methyl chloride styrene Solution was 103 CH temp Polymerization to high 'catalystwas'added to the. solution .whileatheso- .trinsic viscosities is improved through the. use of lumen waswagitated The vcadmlyst; was zarisoluf lower diluent ratios (211 to 411) and highereention of aluminumchloridezin,methyl chloride at centration of catalyst coupled with good 'Qdisa t tio f ojizg per 10 t ,persion, at lower polymerization .temperatures. Solution dd d tthex te ofz oo mL -pgnmin- .However, these variables are interrelated and 11136 until the total pf, catalyst s lution ,present complications; For example, after the added was ml. After aqptenod of about :diluent ratio is lowered the viscosity of the soluminutes, during hi h 9,11 the: catalyst solution 'tion is increased with the result that agitation had been added aboutzzta l of isomonwalco- ..and heat transfer are impaired. Similarly, lOwer- 1101 was added to the 501111 1011 pre- .ing of the polymerization temperature increases cipitate Solid .polymekqs ,frqm .thejsolutignand he viscosity of the Solutio deactivate the catalyst at -103-;C. The excess n p y forming t e high molecular Weight methyl chloride solvent was removed from the polymers that are tough solid resins, certainconpolymer oduct-by.-.vaporization, when the solid ditions are to be fulfilled. The monomeric reproduct Washed with water and dried v.actant is polymerized. efiectively to high molecu- 50 c forlz hours, l d polymer product ,lar weight polymers at temperatures maintained was -1 a t .sofltgning :p int .of su s t a y below a u a d p .40 27 C. and an intrinsic viscosity in toluene.- of 1.4 ably below 100 C. The polymerizationtemt 20 The polymep md t s l ar ,perature may be maintained by use of an inmt in .t 1 The yield 0 by ,ternal or external refrigerant, or both. For exi ht,

.ample Dry Ice (solid'CO2) y be used as an A compilation .of .data..on,a .number of runs in ,internal refrigerant at 78 C. The reaction polymerizingstyrene given in t following may be carried out under ordinary atmospheric table:

'Table I Volume Temp.of 'Time of Type (Catalyst -Vol.-'of Remark I ntiYis of SW2... Home ut Yield eats:

Ml. EC. -Min. H Percent.

-101.-3 4 0.'44 i00 m1- 50 99 1.3 300 -101.- 3' 4 -=d .210 .100 ;1.4 250 101.3 5. v ,1 0, all 1.5 1,000 101.3 5 270 1. 4 72,000 -101;a 7 40 92 1:3 75 101.3 3 .25 .91 1.4 500 -101. 3 '10 165 94 1.5

(Diluent ratio equals 2 vol. MeCl per vol. of styrene).

pressure, but since the reactionis carried out in liquid phase, subatmospheric or superatmospheric pressures may be applied in controlling vaporization of volatile components .inthe .re-

action solution and the temperature of the solu- The catalyst solution may be added inregulated reactant, or the reactant and the catalyst may be brought together in continuous streams to jfOlm a continuous process. The polymerization of the monomeric reactant proceeds upon admixture "of the catalyst. A substantial amount of heatis liberated by the polymerization, but ,by'main- 'taining the reaction solution at'the 'low.po1 y- 75 Injection moldingtemperature, f F 25 O}550 .The @high molecular; weight styrenepolymers 'made at below .'1C., are; tough solid resins notably clear and transparent. ,"These styrene resins have excellent 'moldingcualities and are superior; in some respects to polystyrene molding resins made ,by the high temperature polymerizationmethods. V

A few constantsv on the styrenepolymer moldthe resins produced by thelow temperature caty i po m rizatio areas iollowsz 5. A quantitative comparison of physical properties between the resins made by the low temperature technique and commercial molding resins made by high temperature polymerization are given in the following table:

Table II The factors responsible for the modification of the styrene polymers when they are allowed to remain homogeneously in solution with the active catalyst at a raised temperature are cur.- cult to determine at this time; however, it has been found that the polymers undergo a change in intrinsic viscosity and a change which appears to involve isomerization. The same kind of modification is obtained by treating high molecular weight polystyrene formed at high tempera tures (above 100 C.) with benzoyl peroxide type catalyst. The polystyrene is dissolved in the Friedel-Craits catalyst solution and the isomerization takes place at some temperature above .73 C., e. g., at -24 C. or higher.

The modification treatment may be controlled to obtain a definite grade of resin suitable for blending purposes or other uses. Effects of the modification treatment are illustrated by the following examples:

Table III MODIFICATION TREATMENT Intrinsic VlSCOS- Toluene at 20 C.

Control-:Polystyrene dissolved in methyl chloride, reprecipitated, and mill dried 10 g. Polystyrene Ssame as m the Control A) dissolved in methy chloride with 0.8 g. of aluminum chloride per 100 cc. of methyl chloride solution at 24 C. for 45 minutes. Polymer gerprgcipitated by admixture of acetone and mill 10 g. Polystyrene (same as in the Control A) dissolved in methyl chloride with 0.8 g. of aluminum chloride per 100cc. methyl chloride solution at 24 C. for a perlod in which all the methyl chloride evaporated. Polymer mill dricd Cntl01-P01ysty'rene dissolved in ethyl chloride,

reprecipitated and mill dried 10 g. Polystyrene (same as m the Control D) dis solved in ethyl chloride with 5.04 g. of aluminum chloro bromide (AlBriCl) per 100 cc. of ethyl chloride at 5 C. for 20 minutes. Polymer repregilpitiited by admixture of acetone and mill g. Polystyrene (same as in the Control D) dissolved in ethyl chloride with 7.3 g. of AlBr per 100 cc. of ethyl chlorlde at 5 C. for minutes. Polymer reprecipitated by admixture of acetone and mill dried 10 g. Polystyrene (same as in the Control D) dis.- solved in 600 grams of a 50-50 wt. mixture of methyl chloride-ethyl chloride. 41013 (0.85 g.) was present per 100 cc. of mixed halide at C. for 20 minutes. Polymer repreclpitated by admixture of acetone and mill dried The modification of the polymer by this method. is believed to have certain technical advantages oven direct. formation: of the relatively lower molecular weight polymers, because the modification treatment can be controlled with precision and the molecular weight spread can be kept within a narrow band. For example, if the monomer is polymerized at, say, 25 C'., the resulting polymers have widely varying molecular weights that average to give an internal viscosity of only 0.18, and at 40 C. to give an intrinsic viscosity of only 0.19, but with the modifying treatment a high molecular weight polymer having an intrinsic viscosity above 1.0 may be modified at a temperature above 78 C., e. g., at 25 C., to have any desired intrinsic viscosity in the range below 1.0, e. g., an intrinsic viscosity of 0.9, 0.8, etc. down to 0.2, or lower.

Tests have shown that the modified styrene polymer with a relatively lower molecular weight (internal viscosity reduced to below 1.0) is more compatible with various copolymers of styrene than is the higher molecular weight polystyrene (above 80,000 molecular weight), particularly when the copolymer contains 40% or more of styrene combined therein with an olefin.

Modified styrene resins (made as described in paragraphs B and G of Table III) were compounded on a rubber mill with an isobutylene (40%)-styrene (60%) copolymer in preparing a self-supporting film. Tests showed that the modified resin blended in a proportion of 10% was compatible. The surface tack of the copolymer was reduced. The film showed low moisture vapor penetration.

Similar tests were made by compounding the modified resins (made as described in paragraphs B, C, E, F, and G of Table III) on a warm rubber mill with the following copolymers; isobutylene (50%)-styrene (50%); isoprene (50%)-styrene (50%); butadiene (30%)-alpha methyl styrene (70%); styrene (60%)-dichloro styrene (40%); styrene (20%)-isobutylene (80%); styrene (50%)-isobutylene (40%)-isoprene (10%).

Briefly summarized, the test results indicated that the modified styrene resins with molecular weights in the range of about 30,000 to 70,000 are of special value for blending with the copolymers. Such modified resins have intrinsic viscosities between 0.2 and 1.0.

Also, such modified styrene resins with intrinsic viscosities close to 1.0 but not higher than about 1.3 have suitable fluidity in the region of C. and 225 C. for injection molding, and this region is in the operating range of most commercial injection-molding equipment.

The extent of the modification treatment depends on the length of time that the polymer is maintained in solution with the active catalyst and the temperature at which the solution is held. The modification treatment is abruptly ended by addition of a liquid which precipitates the polymer from the solution, by addition of a I naterial which deactivates the catalyst in the solution, by evaporation of the solvent from the reaction solution, or any combination of such steps. I

A convenient method for conducting the modifying treatment involves mixing the low temperature polymerization reaction solution containing the dissolved high molecular weight polymer with a, diluent, such as propane, in reducing the temperature, and then maintaining the diluted solution at a fixed temperature above 78 C. The diluent vapors may be condensed,

refrigerated, and recycled to the reaction zone. :Inthistype. oitreatment the temperature of the amazes:-

reactiorLsolutiornirray be. maintained, .for: ex ampleyattheboiling point of propane (1-40 C...) under atmospheric pressure or at a-lower temperature under increased pressure with refrige eration with solid carbon dioxide. With methyl chloride itself boiling, the modifying treatment maybeconducted at about 25 C. Atastillv productivity and automaticcontrolc The homogeneous polymerization reaction solution may be passed as a stream continuously through'a reaction zonein a predetermined re action period before separation of the solid polymer' product or before further modification of the polymer product. In this type of operation the low temperature polymerization of styrene differs from other low temperature polymerization reactions in which the polymer becomes precipitated as it is formed.

The modifying treatment described in connection with the polymerization of styrene is also applicable to resins formed by additiveor homopolymerization of styrene derivatives, such as alkyl substituted styrenes in forming commer- -ci ally 'usefulresins, but it isimportant to maintain'the'temperature of the-modifying treatment sufiiciently high to assure homogeneous solution of the resin. The low temperature polymerization of alpha methyl styrene is similar-to that of styrene except that the resulting polymer is less soluble'in methyl chloride at -l C. than is 'th'e'styrene polymer. However, on warming up the reaction solution'to 70 C., all-the alpha methyl styrene polymer becomes dissolved.

Variation in the temperature of formation of the alpha methyl styrene-polymer has*the'-"same effect as in the formation and modification of the styrene polymer. For exampla the alpha methyl styrene polymer prepared "at about 100 C. 'has an intrinsic viseosity of 0.32 or above 0.3, whereas, by raisingthetemperature of the reaction solution to -70 'C'. the intrinsic viscosity-bf the polymer is reduced to 0.13; and the polymer maintained at --25 C. becomes an oil'in-about the same'period'. a g

-- An interesting fact about the alpha methyl styrene polymer is that its softening point is high (190 C., the heat-softening'bar test) despite'its relatively low intrinsic viscosity (0.32).

A styrene. resin ofthe same'intrinsic viscosity has a softening point only slightly above room temperature. With the modifyingtreatment,

useful alpha methyl styrene resinshaving intrinsic'viscosity values/in the range fr0m"0.1 to about 0.3 may be prepared.

Alpha. methyl styrenepolymers.-v differ from polystyrene in other ways; They cannot be alkylatedat low temperatures-and cannotbesulfonated with fuming sulfuric acid under the usual conditions. These 'factsin: addition to those .alreadyxmentioned; namely; thehigh :softening point for acorrespondingnlow" intrinsic viscosity; and the. ineffectiveness. ofrperoxi'decatav.lysts in thermal polymerization,. indicate that talpha rmethyl styrene tends. to z=polymeriz sin a Sucha chain has the aromatic nuclei attached ina-manner which yields ordinary reactions common to a benzene ring, such as sulfonation, alkylation, etc., totake place.

. It appears .that' the alpha. methyl styrene may polymerize to have methylgroups in the alky1- ene linkage.

The profusion of methyl groups'in' sucha structure would tend to give enough steric:.hindrance to prevent the aromatic groups from undergoing the normal reactionsas has. been pointed out to be true of the polystyrene polymers.

The: alpha methyl styrene polymers of high molecular weight were found to be compatible withcopolymers of styrene and olefins and ac-.- cordingly may be used asan addition agentfor raising the heat softening point of such copolymers.

The alpha methyl styrene. polymer resins of suitable plasticity may, also bezused forforming into sheets or extrusion intor-ods.

Using the same procedure. described for the low temperature catalytic formation and modification of styrene and alpha methyl styrene polymers, it was found'possible-to form and modify polymers of alpha-methyl parame'thyl styrene, and. paramethyl styrene.

The alpha methyl paramethyl styrene was polymerized at -103- C. inrnethyl chlorideby use of an aluminum chloride catalyst. The high melting point polymer thus obtained is soluble in the catalyst solution at 25 C., and becomes modified atthis temperature. orhigher temperatures while kept in solution. e

Application of the low temperature catalytic treatment to another alkyl styrene or aryl alkene .isrillustrated in the treatment of polymers of alpha ethyl phenyl ethene or ethyl vinyl benzene: 'The polymers were formed in a, methyl chloride solution of aluminum: chloride, and modified '.at..above. the vaporization. temperature of methyl chloride .in steamldi'stillation.

Other polymers-to which the-"treatment is applicable'in a manner similar to that used .in modifyin polystyrener are polyhalostyrenes, -e.- g.,"pchloro-phenylethene; dichlorophenylethene, pbromophenyl ethene. These polyhalostyrenes are likewise'formed'at temperaturesbelow 78 C. in an alkyl halide solvent with a Friedel- Craftscatalyst and can be kept dissolved. in the solvent with the'catalyst at temperatures ranging from '78-'C.'to 40 c.

It to.-be-. noted that the described-modifying treatment isparticularlyadapted for modifying high molecular weight polymersnformed from styrene (phenylethenel and of itsderivatives which are herein defined as aryl alkenes. 'In these derivatives: the aromatic nucleus or 'aryl radical may containa substituent group, .e. g., a methylor halogen group; also,- the ethene or alkeneside chain-may contain-a substituent; e. g:,

these monomers; aresalso. subject to: the treatment, and are similarly formed by the low temperature polymerization with a Friedel-Crafts catalyst.

The modified polymers have a large number of uses, but are particularly suitable for injection molding to produce such articles as refrigerator parts, battery parts, etc. They may be used together with suitable fillers, dyes, compatible resins, plasticizers or other agents. The modified polymers may be used in coating compositions, adhesives, etc. The relatively low molecular weight polymers retain desirable properties of color, chemical resistance, and heat stability, but are more readily soluble in substances such as drying oils.

The invention is not to be limited to any of the specific examples described, for it is intended to claim all novelty inherent in the invention, including modifications which come within the spirit thereof.

We claim:

1. The method of modifying polystyrene having a molecular weight above about 80,000, which comprises dissolving said polystyrene in a solution containing an active aluminum halide catalyst dissolved in a lower alkyl halide of 1 to 2 carbon atoms, maintaining the resulting solution at a temperature of -25 C. to 5 C. a sufficient time for the dissolved polystyrene to undergo modification with decrease in its molecular Weight to about 30,000-70,000, and thereafter precipitating the thus modified polystyrene from the solution.

2. In the method described in claim 1, said aluminum halide being aluminum chloride, and said solvent being selected from the class consisting of methyl chloride, ethyl chloride, and methylene chloride.

3. The method of modifying a polystyrene resin having a molecular weight above 80,000 and an intrinsic viscosity, above 1.0 in toluene at 20 10 C.-, to reduce its molecular weight and improve its compatibility with styrene-olefin copolymers having at least 40% combined styrene, which comprises dissolving said polymerized polystyrene in a solution containing aluminum chloride dissolved in a lower alkyl halide of 1 to 2 carbon atoms, maintaining the resulting solution at a temperature of 25 C. to 5 C. for a sufiicient time to reduce the intrinsic viscosity of said polystyrene to the range of 0.9 to 0.2, and thereafter precipitating the resulting modified polystyrene from the solution.

4. The method of modifying a polystyrene resin having an intrinsic viscosity of about 1.17, to reduce its molecular weight and improve its compatibility with styrene-isobutylene copoly mers having about 40 to combined styrene, which comprises dissolving 10 grams of said polystyrene in 600 grams of a mixture of equal parts by Weight of methyl chloride and ethyl chloride containing 0.85 gram of aluminum chloride dissolved therein per cc. of mixed chloride solution, and maintaining said solution at -25 C. for 20 minutes, and precipitating the resulting modified polystyrene having an intrinsic viscosity of 0.84 from said solution.

WILLIAM J. SPARKS.

DAVID W. YOUNG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 785-787 (May 1945) 

1. THE METHOD OF MODIFYING POLYSTYRENE HAVING A MOLECULAR WEIGHT ABOVE ABOUT 80,000, WHICH COMPRISES DISSOLVING SAID POLYSTYRENE IN A SOLUTION CONTAINING AN ACTIVE ALUMINUM HALIDE CATALYST DISSOLVED IN A LOWER ALKYL HALIDE OF 1 TO 2 CARBON ATOMS, MAINTAINING THE RESULTING SOLUTION AT A TEMPERATURE OF -25* C. TO 5* C. A SUFFICIENT TIME, FOR THE DISSOLVED POLYSTYRENE TO UNDERGO MODIFICATION WITH DECREASE IN ITS MOLECULAR WEIGHT TO ABOUT 30,000-70,000, AND THEREAFTER PRECIPITATING THE THUS MODIFIED POLYSTYRENE FROM THE SOLUTION. 